The production of neutrophilic granulocytic leukocytes in the bone marrow involves a process by which immature progenitor cells committed to the granulocytic line of development undergo proliferation to produce morphologically recognizable precursor cells. The latter further undergo a final phase of nuclear and cytoplasmic maturation which ultimately leads to a highly differentiated, non-dividing, end-stage granulocyte that eventually leaves the marrow and circulates in the blood. Although considerable information has been obtained in recent years about the specific factors that regulate the proliferation of granulocyte progenitor cells,.sup.1/ much less is known about the factors that regulate the maturation of the recognizable granulocyte precursor cells.
Using a liquid culture assay system, we showed previously that the end-stage of neutrophilic granulocyte precursors obtained from guinea pig bone marrow could be induced in vitro by dialyzed normal guinea pig serum..sup.2/ We also showed that this maturation-inducing activity could be attributed to the transferrin fraction of this serum and that purified human serum transferrin could be substituted for guinea pig transferrin as a maturation inducer for guinea pig cells..sup.3/